The present invention relates to an optical recording medium, and more particularly, to an optical recording medium which is compatible with compact discs (CD's), has a high reflectivity at wavelengths longer than 600 nm, and in and from which information can be recorded and reproduced by a digital versatile disc player (DVDP) and a compact disc player (CDP), and further can be produced at low costs.
The recording area per recording unit of an optical recording medium is smaller than that of a conventional magnetic recording medium, so that the optical recording medium has been used as a high density recording medium. Optical recording mediums are classified into the following types: read only memory (ROM) type which only reproduces already recorded information, write once read many (WORM) type which is recordable just once and readable many times, and rewritable type which is recordable, erasable and rewritable medium. The information recorded in the optical recording medium should be reproduced by a player for the ROM type medium. To this end, the optical recording medium must satisfy a conventional standardization rules (RED BOOK), that it have a reflectivity of 65% or more and carrier-to-noise ratio (CNR) of 47 dB or more.
In a recordable optical recording medium, the recorded information can be reproduced based on the change of the reflectivity which is caused by a physical modification, phase change or change of a magnetic property in a recording layer before and after the recording. Also, for making the optical recording medium compatible with CD's, a long-term data storing property and a high recording density are required as well as the above high reflectivity and CNR characteristic. In order to improve the characteristic of the optical recording medium and make the manufacturing process easily, various optical recording media being formed of various materials have been suggested, and a group of them is in practical use.
As a conventional optical recording medium, Japanese Laid-open Patent Publication No. showha 63-268142 discloses a recording medium having a structure in which a sensitizing layer formed of gelatin, casein or polyvinyl alcohol (PVA), and a metal thin film as a recording layer formed of chromium (Cr), nickel (Ni) or gold (Au) are sequentially stacked on a substrate. According to the optical recording principle of the recording medium, the metal thin film absorbs heat of an irradiated laser beam, and thus the sensitizing layer and the metal thin film are deformed to form a recording pit. However, the recording pit of this recording medium is exposed so that it is difficult to store the recorded information for a long-term.
U.S. Pat. No. 4,973,520 discloses a recording medium having a structure in which three metal thin films are stacked on a substrate. The recording characteristic of the recording medium is good. However, the long-term storage of the recorded information cannot be achieved due to its exposed recording pits.
In order to solve the above defect, U.S. Pat. No. 4,983,440 discloses a recording medium having a structure in which two metal thin films which act as a recording layer, and a protecting layer for protecting the recording layer are sequentially stacked on a substrate. However, this recording medium has a very low reflectivity of about 20% and requires a high-power light source for the practical use. Also, the recording medium cannot be compatible with a conventional CD.
According to U.S. Pat. No. 5,328,813, a metal thin film as a recording layer is formed on a substrate, and a hard metal oxide layer is formed on the metal thin film to increase an information storing property and the reflectivity to 40.about.69%. However, the CNR is still low.
Also, U.S. Pat. No. 5,155,723 discloses a recording medium having a structure in which an organic dye layer as a recording layer is stacked on a substrate, and a reflecting layer and a protecting layer are stacked on the recording layer. According to the recording medium, the dye layer absorbs a recording laser beam for a recording to emit heat. Also, the substrate is heated and deformed by the heating, so that a recorded signal can be reproduced based on the difference of the reflectivity according to the recording. Here, the reflectivity of the recording medium is 70% or more and the CNR after recording is 47 dB or more to be compatible with the CD. However, the recording layer of the recording medium has low resistance against heat and light, and the manufacturing costs are high due to the use of expensive organic dye. Also, a spin coating is performed during the manufacturing process with an organic dye solution. However, the thickness of the coating layer must be accurately controlled with a deviation of .+-.3% or less during the manufacturing of an optical disc since the reflectivity is dependent on the thickness of the coating layer. Also, costly equipment is required and productivity is low.